Grounding conductors (also termed earthing conductors according to US nomenclature) are well known in the art, where this term herein means a grounding conductor intended for air and/or ground laid power line distribution network applications, where the grounding conductor is adapted to be laid underground, in water or in air, outside or inside ducts; and where the grounding conductor is provided in connection with or at a distance to an AC or DC power cable, such as an AC single or three phased power cable, or power structure, such as a pole, a transformer station, or a building, which power cable or structure is provided above ground, or provided in direct contact with or alongside said power cable in the ground. The purpose of the grounding conductor is to form a ground or earth potential to the power cable or structure. Some grounding conductors are provided following, along, or being integrated with ground laid power cables. In some cases at least part of said grounding conductor is provided following, along, or integrated with aerial power cables. The grounding conductor can be applied in the ground near individual poles, on site providing system ground, under transformers, or close by buildings, e.g. by providing a mesh, or can be applied following the power cable and laid underground as a ground conductor. Said conductor can be utilized comprising any suitable diameter as to form straight or twisted wires, strands, rods, or pipes.
Further, the term “grounding conductor” is herein defined as a conductor suitable for being in electrical contact with the surrounding environment for providing grounding potential along substantially the entire length of the grounding conductor where laid in the ground. Thus, the term “grounding conductor” does not cover externally electrically insulated and internally electrically conducting conductors which only provide grounding potential at the conductor end or ends with e.g. a cable lug or clamp.
A bare copper material is conventionally used as a grounding device. Bare copper materials serving as grounding devices may be arranged on or in the earth buried at a predetermined depth, and the copper materials being arranged in a grid pattern in a vertical and horizontal directions. This constitutes a mesh electrode. By way of example, a grounding device having such a mesh electrode is disclosed in the Unexamined Japanese Patent Application Publication (Tokkai-Sho) No. 59-211975.
However, when the conventional grounding device exhibiting a bare metal surface such as bare copper e.g. constituting a mesh electrode as described above, said bare metal devices are arranged directly in contact with and inside the soil of the earth. Accordingly, corrosion of the bare metal grounding device is facilitated by moisture in the soil, the acidity of the soil, the concentration of salt in the soil, and the like. Thus, disadvantageously, the buried grounding device can be damaged thereby and may be subjected to a disconnection or the like, thus spoiling the grounding function of the grounding device.
Generally, bare copper grounding devices tend to exhibit a long lifetime due to its good inherent corrosion stability. However, other metals are more easily corroded, such as e.g. aluminium and iron.
If the bare metal is covered with a low resistance material comprising a compound of electrically conducting carbon material and a binder material, often called a cement, thus providing a grounding conductor, a disconnection or the like caused by the corrosion attributed directly to the soil is of course less likely to occur as compared to when the bare metal, such as the aluminium or iron arranged directly in the earth.
However, the specific resistance of the soil may change rapidly locally due to environmental or soil conditions, such as lightning strikes or water content, or it may change over the length of the grounding conductor. Thus, in areas in which the grounding conductors are not completely covered with low resistance material, e.g. has holes due to e.g. mechanical damages or lightning induced damages, which holes lead to one or more areas of exposed bare metal. In these areas electromagnetic leaders may form and extend from the ground conductors during such lightning strikes, as the carbon in the covering compound is very unlikely to be ionized. Consequently, in an electrolytic state such as the one in the soil, contact with the bare metal surface areas causes such conventional grounding conductors to be corroded by a battery action resulting from a difference in potential between ground and metal strand in these areas, leading to a direct current flowing here, causing what is called galvanic corrosion. When exposed to particularly disadvantageous environmental or soil conditions, such damaged grounding conductors can then experience even very fast galvanic corrosion, even and especially when only tiny holes are provided, which may lead to malfunction, disconnection, or the like.
The rising cost of copper materials and poor social conditions in some parts of the world have made it a lucrative business for some people to steal copper containing objects, such as grounding conductors. This leads to the loss of ground connection of the power cables, which may lead to equipment failure(s) and—more seriously—a risk for living beings being exposed for lethal electric shock. The purpose of the grounding conductor, in combination with other protective devices such as fuses and residual current devices, is ultimately to protect and ensure that living beings do not come into contact with metallic or leading objects, such as a power line or the ground itself, whose potential relative to the living beings own potential exceeds a “safe” threshold, typically set to around 50 V.
Further, the rising copper costs and the stolen copper also encourage suppliers and operators of power lines to consider reducing the amount of copper materials being utilized, in particular when installing or re-installing longer lengths of grounding conductor along a power cable.
It has been attempted to substitute all or at least part of the copper material of the grounding device or conductor with less costly materials, such as iron, aluminium, copper clad aluminium, magnesium, or even tin. Even though aluminium inherently—on contact with oxygen—forms a thin layer of aluminium oxide which generally tend to lower surface corrosion, aluminium does tend to be more reactive with water than copper. Further, aluminium is less electrically conductive, around 40-60% less conductive. Thus, aluminium conductors generally are made with a corresponding larger diameter in order to attain the same resistance as the copper ones.
EP 0 136 039 dating from 1983 describes a grounding device in the form of a ground rod or mesh mat intended for lightning protection by burial in the ground, where the device comprises a metallic central member, e.g. made of aluminium or preferably steel, and a surrounding electrically conductive polymer member for moisture protection. The polymer member has a specific resistance of from 0.01 Ωcm to 10 Ωcm.
JP 10-223279 dating from 1996 describes a method of preventing the oxidation of a grounded electrode made from copper, aluminium or steel by using a semi-conductive anticorrosive layer formed on a conductor as the grounded electrode to be electrically connected to electric equipment and embedded in the ground. The semi-conductive anticorrosive layer is formed by covering a crosslinked or non-crosslinked semiconductive polyethylene composite with a semi-conductive resin composite containing conductive carbon black by extrusion, and its volume specific resistance is set to 1/20- 1/100 of the specific resistance of the soil embedding the electrode.
Contrary to what one would expect, an increased use of aluminium grounding devices has yet to be seen, since no commercially accepted solution has been provided, in particular the problem of corrosion protecting aluminium surfaces has not been dealt with sufficiently.
Inventor has previously in several tests and attempts tried to find a solution by applying one or more twisted aluminium strands with conductive resin, such as the polyethylene-based semi-conductive jacketing polymers sold under the trade number LE0563 from the company Borealis and trade number DHDA 7708-BK from company DOW. But Inventor has hitherto failed to produce any suitable solution where the polymer was sufficiently protective against water and leak current corrosion, at least when damaged, while at the same time providing a suitably conductive or grounding capable grounding conductor.